The present invention relates to a process and apparatus for generating a pesticidal gas mixture of phosphine gas and air for fumigating an enclosed environment with phosphine.
Phosphine gas has been used for decades as a fumigating gas for disinfesting closed environments infested with or suspected to be infested with pests, including higher animals such as rodents, but particularly insect pests such as weevils, bugs, moths and cockroaches, either mature or in various larval stages or in the form of eggs. Such fumigation is particularly used for the disinfestation of agricultural bulk commodities such as grain, legumes, e.g. soya, beans and peas, groundnuts, various other kinds of nuts and seeds, Godder, tobacco, plant and animal fibres and many other commodities. Most commonly such fumigation is carried out by metal phosphide-containing preparations, e.g. tablets or pellets or powders or granulates packed in satchets being added directly to the commodity or into the closed environment. The moisture content of the commodity or air in the environment reacts with the metal phosphide resulting in the generation of hydrogen phosphide in the commodity or environment. This procedure suffers inter alia from the following disadvantages:
a) the phosphine concentration pattern cannot be influenced. As a function of temperature and humidity the phosphine gas concentration initially rises more or less steeply up to a maximum and from there drops asymptotically to zero at a rate which depends on phosphine losses due to leakage, decomposition or other causes. In extreme cases this may result in the phosphine concentration dropping so rapidly that complete killing of the pests, in particular also in their preadult stages, cannot be ensured. As a general rule it is preferred in phosphine fumigation to maintain relatively low pesticidal gas concentrations as constantly as possible over a prolonged fumigation period rather than employing high concentrations for shorter periods, the latter procedure being generally less effective. Accordingly, it would be desirable to be able to regulate the phosphine gas concentration pattern during fumigation and to maintain the desired lethal concentrations or pattern of concentrations over a prolonged period of fumigation by the controlled addition of phosphine gas. PA1 b) The metal phosphide preparations leave behind relatively harmless inert decomposition residues. However, these residues usually also have greater or lesser relatively minor residual contents of undecomposed metal phosphides. These residues remain in the commodity and must be removed eventually by aspiration or by recollecting the spent sachets (or equivalent packaged forms of the pest control agent). PA1 a) In U.S. Pat. No. 4,200,657 a process is described wherein phosphine gas is generated by the decomposition of metal phosphide pellets or tablets, whilst the air to which the phosphine gas is released .is slowly circulated through grain or like commodities contained in a silo equipped with gas circulation means. The disclosure mentions the possibility of introducing the metal phosphide composition anywhere in the system, including the gas circulation ducts, however, in practice this method is carried out invariably with the metal phosphide compositions widely scattered over or inside the commodity to be fumigated, because of the risk that otherwise high local concentrations of phosphine might be formed which could easily exceed the autoignition concentration. This latter problem is also addressed in an improvement of that process forming the subject of U.S. Pat. No. 4,651,463. There as well, the gas generation generally is carried out in the same space which also contains the commodity to be fumigated. PA1 b) The external generation of phosphine gas by the hydrolysis of metal phosphides, e.g. using liquid water, is also suggested in ZA-PS 80/7479. According to that disclosure specific additives are employed in the metal phosphide compositions designed to reduce the violence of and to slow down the hydrolysis reaction thereby to reduce the risk of autoignition. This particular proposal is applicable only to the hydrolysis of aluminium phosphide and is ineffective, for example with magnesium phosphide. Moreover, the additives there disclosed are sometimes undesirable. PA1 c) ZA-PS 86/4806 discloses the fumigation of grain in silos using bottled mixtures of phosphine and inert gases such as nitrogen or carbon dioxide kept in steel cylinders and passsed from such cylinders by pressure release into the silo cells. That process is expensive because it requires the keeping of stocks of expensive gas steel cylinders equipped with valves made of such materials that they are not attacked by phosphine gas. The bottling itself is expensive, and particularly because the steel cylinders preferably contain at the most 3% by volume of the phosphine gas. Also, the manufacture of and handling of bottled phosphine gas is somewhat hazardous and can be carried out only at centralised factories from where relatively large numbers of gas-filled bottles have to be transported to the fumigation site. PA1 d) Finally, it has been proposed, in German patent application P 36 18 296.4 to generate phosphine, preferably continuously by hydrolysing the metal phosphide with an inert fluid, e.g. carbon dioxide, containing water. As the inert fluid flows through the phosphide, phosphine gas is generated which is then passed into the commodities to be fumigated. This procedure lends itself also to carrying out the phosphine generation at the fumigation site. However, this procedure also suffers from the disadvantage that relatively large quantities of the inert fluid have to be transported in sealed cylinders or in the form of dry ice to the fumigation site.
Accordingly, various proposals have been made or become known in the past, whereby it is said to be feasible to generate phosphine gas from metal phosphides externally of the environment to be fumigated and to feed the gas so generated into the enclosed enviroment, respectively into the commodities therein contained: